1. Field of the Invention
The present invention relates to a dispersion-type liquid crystal electro-optical device comprising a liquid crystal/resin composite comprising a high polymer resin having dispersed therein a liquid crystal material. More particularly, the present invention relates to a reflection-type liquid crystal electro-optical device having high light transmittance and capable of providing images with clear black.
2. Description of Prior Art
Liquid crystal electro-optical devices well known and already put to practice heretofore are those operating in TN (twisted nematic) mode or STN (super twisted nematic) mode, in which nematic liquid crystal compositions are used. Recently, liquid crystal electro-optical devices taking advantage of ferroelectric liquid crystals are also realized. The liquid crystal electro-optical devices above basically comprise a first and a second substrate each having provided thereon an electrode and a lead, and a liquid crystal composition having incorporated therebetween. Thus, the liquid crystal composition can undergo a transition between states by applying thereto an electric field through the electrodes provided on the substrates. These changes in states are ascribed to the anisotropy of the dielectric constant of the liquid crystal composition itself in the case of nematic liquid crystals, etc., and to the spontaneous polarization in the case of ferroelectric liquid crystals. In this manner the electro-optical effect due to the changes in state of the liquid crystal molecules can be utilized to give an electro-optical device.
In the TN mode or the STN mode liquid crystal electro-optical devices, the liquid crystal molecules within the plane of the liquid crystal layer in contact with the substrate arrange themselves along the rubbing direction upon applying a rubbing treatment to establish a molecular orientation. The upper and the lower substrates are displaced from each other in such a manner that the rubbing direction of one substrate make an angle in the range of from 90.degree. C. or from 200.degree. to 290.degree. C. to that of the other. Thus, at the central portion of the liquid crystal layer, the liquid crystal molecules are arranged in a spiral to minimize the energy between the upper and the lower liquid crystals which are positioned with respect to each other within an angle in the range of from 90.degree. to 290.degree. C. Furthermore, in such a construction, the liquid crystal material in an STN mode device may be a mixture with chiral substances if necessary.
For the known electro-optical devices above, however, it is requisite to incorporate polarizer sheets and also to maintain the liquid crystal molecules in a regularly oriented manner within the liquid crystal electro-optical device. The treatment for establishing a molecular orientation comprises rubbing the orientation film (which is an organic film in general) with a cotton cloth or a velvet cloth. If no such treatment is applied, the electro-optical effect of the liquid crystals cannot be expected because no uni-direction oriented liquid crystal molecules would be realized. Accordingly, the device inevitably comprises a pair of substrates to define a space to maintain therein the liquid crystal material. Thus, the liquid crystal is injected into said space and then subjected to orientation treatment to realize an optical effect.
In contrast to the liquid crystal electro-optical device above, there is also known a dispersion-type liquid crystal which can be employed free of such polarizers and robbing treatment, and which yet provides an image plane having a brighter contrast. The light control layer of this dispersion-type liquid crystal comprises a light-transmitting solid polymer maintaining therein the liquid crystal material in granules or in a sponge-like structure. The liquid crystal device can be fabricated by dispersing encapsulated liquid crystal material into a polymer, and then providing said polymer on a substrate as a film or a thin film. The liquid crystal can be encapsulated with gum arabic, poly (vinyl alcohol), gelatin, and the like.
In a dispersion-type liquid crystal comprising liquid crystal molecules encapsulated with poly(vinyl alcohol) and having a positive dielectric anisotropy, for example, the liquid crystal molecules arrange themselves in such a manner that the major axes thereof become parallel to the direction of the electric field. If the refractive index of the solid polymer is equivalent to that of the arranged liquid crystal upon application of the electric field, the light control layer turns transparent. When the electric field is turned off, the liquid crystal molecules take a random arrangement and hence the refractive index thereof greatly deviates from that of the solid polymer. Thus an opaque state is realized, because the light is scattered by the liquid crystal molecules and the light transmittance becomes low. The device takes advantage of the difference between the transparent state and the opaque state to provide information of various types.
In addition to the encapsulated type, dispersion-type liquid crystals include those comprising liquid crystal materials being dispersed in an epoxy resin; those taking advantage of phase separation between the liquid crystal and the resin, which is realized by irradiating a beam onto a mixture of a liquid crystal and a photo-curable resin to cure the resin; and those comprising a three-dimensionally bonded polymer impregnated with a liquid crystal. In the present invention, the term "dispersion-type liquid crystal" encompasses all the types enumerated above.
The dispersion-type liquid crystal electro-optical devices above are freed from polarizer sheets and hence have extremely higher light transmittance as compared with those of the conventional electro-optical devices operating in a TN mode, STN mode, etc. More specifically, the transmittance per single polarizer sheet is about 50%. Hence, in an active matrix type electro-optical device using a combination of said polarizer sheets result in a final transmittance of about 1%; in an electro-optical device operating in an STN mode, the actual transmittance is about 20%. Accordingly, much effort in those conventional electro-optical devices is placed to realize a bright display by increasing illuminance of the back-lighting. The dispersion-type liquid crystal electro-optical devices transmit, in contrast to the conventional electro-optical devices, 50% or more of the incident light. This is a unique superiority of the dispersion-type liquid crystal electro-optical devices which results from their structure free of any polarizer sheets.
As mentioned earlier, a dispersion-type liquid crystal takes a transparent state and an opaque state, and because it is capable of transmitting a large amount of light, research and development efforts are generally concerned in realizing a transmitting type device. Particularly among them, projection-type liquid crystal devices are the most actively developed types. A projection-type liquid crystal electro-optical device comprises a liquid crystal electro-optical device panel placed in the light path to intervene the light beam emitted from the light source, so that the light having passed through this panel may be projected on a wall plane through a slit provided at a predetermined angle. The liquid crystal molecules in this panel are in a random arrangement at a low level electric field below the threshold value in which the liquid crystal molecules do not respond, and hence provides a white opaque state. The light incident to the panel at this instance is scattered upon passing through the panel to largely extend the light path thereof. Accordingly, the scattered light is mostly cut off by the slit provided subsequent to the panel. A black state realizes on the wall by thus cutting off the scattered light. When an electric field is applied at an intensity over the threshold value, on the other hand, the liquid crystal molecules arrange themselves in response to the electric field to make a parallel arrangement with respect to the direction of the electric field. Thus, the light incident thereto advances straight without being scattered to finally realize a bright state with high luminance on the wall.
Three dispersion-type panels each equipped with corresponding one of red, green, and blue color filters are established to obtain a synthesized image from red-colored image, green-colored image, and blue-colored image. When the three colors are transmitted and superposed on the wall, the image obtained as a result is a white and bright image. On the contrary, if the three colors are in a scattered state, the resulting image expressed on the wall is a black one.
As mentioned in the foregoing, a black and a light-transmitting state can be obtained with a dispersion-type liquid crystal electro-optical device operating in a projection mode. The use of a slit at a predetermined angle to realize black images, however, disadvantageously reduces the amount of transmitted light. Thus, the liquid crystal electro-optical device thus obtained could only provide a slightly lighter image even in a direct view type one.
A dispersion-type liquid crystal electro-optical device can be also operated in a reflection mode. However, the reflection mode electro-optical device realizes a white state but no black state.